Effortless beauty

Susan J. Wiegrefe

A key to growing low-maintenance trees is performing the critical
preplanting work of assessing the site's limitations and selecting plant
material accordingly. Matching a tree species' strengths and tolerances to
the site conditions, especially on sites with extreme conditions, can mean
the difference between the plants' languishing and flourishing. While some
species can adapt to a variety of growing conditions, it often is necessary
to search out a "specialist" to fill the position when extreme growing
conditions or multiple plant-stress factors exist. This especially is true
if you expect strong branch patterns and aesthetic appeal, in addition to
mere survival.

Some trees are generally adaptable, while others specialize in some extreme
condition, such as high or low pH or soil moisture. In many instances,
breeders select cultivars for specific combinations of traits, often
aesthetic and tolerance to stresses. Because of their superior performance,
these cultivars generally are worth the slight additional cost and effort
to locate them.

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Trees may possess other qualities, independent of their suitability for a
specific site, which qualify them as low-maintenance species. Five criteria
dictate, to a large extent, the level of maintenance you'll need to perform
to produce a healthy, attractive specimen over the course of many years.

1. Most importantly, the tree should be free of disease or insect pests
that can endanger or disfigure the tree if you leave them untreated.

2. The tree must not require annual pruning such as the removal of water-
or basal-sprouts to maintain the desired form.

3. It should not produce excessive fruits. Recognizing that trees, like
most plants, require flowering and fruiting for perpetuation of their kind,
we must accept some litter. However, excessive litter is offensive and may
increase maintenance costs. Thus, a list of low-maintenance trees should
only include species with which this "problem" is minimal.

4. The tree should be strong-wooded and have strong branching patterns so
that it does not require cabling or substantial pruning to remain solid
even when mature.

5. Lastly, it should tolerate climatic conditions sufficiently that it does
not depend on neighboring vegetation or structures to provide acceptable
growing conditions, such as wind protection, shade or any other
microenvironment.

Every recommended-tree list should apply to a specific geographic region
and its general climatic and soil conditions. In this case, my
recommendations apply to the North Central states. In this region, erratic
rainfall and alkaline soils combine with cold winters and hot summers to
make life difficult for woody plants. Wherever you're located, the efforts
you make (proper planting and care through the tree's establishment period)
to moderate the challenging conditions existing on the site will reward you
with healthy young trees that require minimal long-term maintenance.

Below are some excellent tree species that fit the criteria of "low
maintenance" and possess excellent ornamental value as well. The table on
page 17 summarizes characteristics of these species.

Minimum-maintenance tree species
* The three-flowered maple is an excellent choice if you are looking for a
smaller tree with outstanding fall color and tolerance of drier sites. Its
rounded canopy ultimately reaches to 30 feet with equal spread. Its leaves
reliably turn orange, often with a blush of red or maroon. In the upper
Midwest, this Manchurian species outperforms its close relative, the
lacebark maple (Acer griseum), which originated in central China. The
three-flowered maple is hardier (to USDA Zone 4) and tolerates heavier and
slightly more alkaline soil than its kin. Another common name for this
species, shaggy-bark maple, describes yet another of its attractions: The
grayish-tan bark exfoliates on branches 3 years old or older.

This species has been difficult to find in the trade. Its scarcity is due,
in part, to the difficulties of propagating it. As domestic seed orchards
begin to bear, this problem should ease. However, its slower growth rate
relative to the more common maples and ashes will continue to make it
somewhat more expensive to produce. Although mass plantings of this species
may not be feasible, its many strengths and charms will undoubtedly earn it
a place in many locations as it becomes better known.

* The Freeman maple fills a key role at the other end of the moisture
gradient. The hybrid between the red and silver maple is one of the best
choices for sites that periodically flood or drain poorly. The Freeman
maple is a good example of interspecific h ybrids that exhibit the
complementary strengths of their parents. Tolerance of flooding and
alkaline soil are attributes contributed by silver maple. The red-maple
component improves fall color and moderates the rank growth of limbs and
roots usually associated with silver maple.

A number of selected cultivars are dioecious (individual trees are male or
female, but not both--a red-maple trait), thus allowing for the selection
of seedless, male clones. Consistently brilliant fall color has been the
primary selection criterion for most cultivars, but they also differ in
growth habit.

* American hornbeam or musclewood presents an option for providing
brilliant fall color on a smaller scale. It occurs naturally as an
understory species, rarely exceeding 30 feet in height. It often is broader
than tall and forms multiple trunks, playing up its beautiful, smooth bark.
The tight-fitting bark in conjunction with fluted or corded branches are
evocative of well-muscled human limbs, hence the common name of musclewood.
Although tolerant of shade, the canopy does not grow as dense nor does the
fall color develop as fully as when it grows in full sun. The key to
keeping this species healthy and low-maintenance is to place it where it
receives sufficient moisture.

Regardless of whether it develops fall color, this species' summer
character is reason enough to use it. Its growth habit is refined, and its
fine-textured foliage is interspersed with pendulous clusters of fruit.
These 2- to 4-inch-long "Chinese lanterns" also change color with the
seasons and may persist after leaf drop. The amount of flowering and
seedset varies considerably from year to year.

* Cornelian cherry dogwood is another smallish, often multiple-trunked tree
deserving greater use. It is quite versatile, growing in a variety of soil
types and pH levels, and in full sun or partial shade. Its long-lasting
clusters of cheerful yellow flowers (see photo, above left) are welcome in
early spring when they appear with or even before the earliest forsythias.
The leaves are a glossy, dark green that combine beautifully with the
bright-red fruit, which ripen in July. The bark is not quite as attractive
as the closely related (and less-available) Japanese Cornelian cherry
dogwood (Cornus officinalis), but the tastier fruits and less-congested
branching pattern compensate for this. By choosing cultivars selected for
their tree-like tendencies and properly pruning young trees, your specimens
will become delightful small trees rather than large shrubs.

* The Turkish treehazel has experienced a rise in popularity lately, but
designers often still ignore it in many situations where it would be an
asset. This low-growing member of the birch family is tolerant of drought,
heavy soils, extremes in temperatures and a broad range of soil pH. Its
horizontal branching and tough wood make it resistant to wind and ice
damage. Its ornamental features include its stately pyramidal shape,
coarsely furrowed bark and the early spring display of its pendulous,
yellow, male catkins. In the more northerly portions of its range, low
temperatures may kill the flowers--even though its vegetative portions are
completely hardy--eliminating any catkin display or nut production.

Hazels do not set seed if self-pollinated and rarely do so with pollen from
a sibling. Thus, unless you plant unrelated individuals together, they will
produce few of their tasty nuts. However, you can exploit this trait if you
prefer to avoid dealing with the interesting, but spiny, nut clusters.

Unlike many nut trees, this species doesn't form an extensive tap root and
transplants relatively easily if you provide it with supplemental water
during its first couple of years of establishment. Difficulties in
propagation result in lower availability and higher cost. Its durability
and elegant presence, however, make it well worth the effort to acquire and
establish.

* Ironwood or American hophornbeam, another member of the birch family, is
in many ways a smaller version of the Turkish treehazel. Although it
doesn't take heavy soils quite as well as the treehazel, ironwood tolerates
drought, wind, ice and temperature extremes. It grows slowly, starting out
with a pyramidal form, but fills out to become more rounded with maturity.
The bark is fine textured, peeling off in thin vertical strips.

Its flowers and fruits are quite different from the Turkish treehazel,
however. The male catkins are clustered in groups of three at the branch
tips instead of being distributed individually. The nutlets are small and
borne in an elongated cluster, each encased in a papery pouch. Hairs that
can irritate the skin cover these fruit clusters.

Another characteristic of note is the observation that deer browse on
ironwood casually but do not prefer it as a forage. In areas where large
deer populations threaten unprotected plantings, this feature should prompt
landscapers to take a closer look at this species.

* Callery pear possess several strengths that make it valuable in tough
urban settings. 'Bradford' callery pear has been planted widely, and the
cultivar name became synonymous with the species to many people. When many
'Bradford' pears reached maturity and began to break apart due to poor
branching structure, the entire species got a bad name. However, breeders
have selected cultivars of callery pear with improved branching structure
as well as better cold hardiness (another weakness of 'Bradford' pears).

The species tolerates a wide range of soil conditions including drought and
poorly drained, low-oxygen conditions. In addition, its flower display,
glossy foliage, breath-taking fall color and uniform growth habit give it
ornamental value in all seasons. The problems of thorniness, fireblight
susceptibility and fruit litter--serious in some other pear species--are
minimal in callery pear but vary with the cultivar. Cultivars are available
with another desirable trait for planting sites of limited size: a narrowly
upright growth habit (see photo, page 14, middle).

Be aware when choosing a cultivar that some selections assume their fall
color earlier than others. This is especially important for northern sites
where frost can occur before fall color has developed on the late
cultivars. If you intend to use callery pear in the North, be sure to
choose a cultivar that colors relatively early in the fall.

* My last selection is 'China Peking lilac (Syringa pekinensis 'Morton' or
'Watertower'). Besides meeting low maintenance criteria, this species is a
prime example of a tree with highly ornamental bark. Its interesting bark
color ant texture increase its attractiveness year round but are especially
nice during the Midwest's long winters. The bark starts out with a mahogany
color that appears to be polished smooth, sprinkled with corky, beige
lenticels. It adds greater interest with age when it begins to peel
horizontally (see photo, page 17), similar to paper birch. Another
ornamental feature is the large clusters of cream-colored, lightly fragrant
flowers that bloom in June (in Northern areas). The species contains
considerable variation in bark character, but breeders have selected 'China
Snow' for its consistently ornamental bark.

No one has yet determined the northern limit of this recently introduced
cultivar. Though the species is generally less winter-hardy than the
closely related Japanese tree lilac (Syringa reticulata), certain seed
sources are hardy at the Minnesota Landscape Arboretum. In addition, data
from the Morton Arboretum indicate that it is more drought-tolerant than
its Japanese relative. Although perhaps not appropriate for the toughest of
sites, this selection will add summer bloom and year-round interest to a
well-drained sunny site with minimal maintenance.

Dr. Susan Wiegrefe is a research tree breeder for the Morton Arboretum,
Lisle Il